Contents of the DRIVEBUY.TXT file

Buying a Hard Disk

written for the CPCUG build 1/1/91

Selecting a hard disk is an important question in buying a newcomputer and a continuing question in upgrading existing IBM-PCcompatible computers. This document provides background data sothat users can assess tradeoffs between price, performance, andfuture upgrade costs, and make better decisions. This document consists of two parts. The first part waswritten by Mike Focke and covers general principles underlying harddisk function, describes some guidelines which may not apply to afew PC compatible computers but do apply to the vast majority, andcontains some specific recommendations. The second part of thisdocument was written by Doug Wagner, and contains furtherexplanation of some of the tradeoffs and options, along with tablesdescribing speed, size, and cost of a larger number of currentlywidely available hard disks. It is our hope that this description will be useful to bothbeginners and to advanced users.

Buying a Hard Disk

ST506, MFM, RLL, ERRL, ARRL, ESDI, SCSI, and AT/IDE

What do they mean? Which should I buy?

written for the CPCUG build 1/1/91 by Mike Focke

Lets start by defining what are the important questions to ask whenyou go to buy a hard disk subsystem.

1. How much data can it store?

2. How quickly can it move the mechanical components and begin transferring data?

3. How fast can it transfer data once it has begun?

4. How reliable will it be?

5. How much does it cost?

6. What comes with it?

7. What if it breaks?

If you have the answers to these questions, you can make aninformed choice based on your projected needs.

So this paper will discuss items you should know as background whenasking these questions and will make some recommendations ofseveral subsystems known to work well together and to be easilyinstalled.

For the purposes of this discussion, I am going to assume that youare buying a new hard disk subsystem to go in the new 386SX or DXmachine you are going to build.

If you are going to use parts from another machine, then your mostimportant issues are simple things like do you have a slot thatwill fit the controller and will the drive mount in your case andare the cables long enough. I would suggest you read on aheadbecause you may find out things that persuade you to buy a newsubsystem and sell your old one. Over the last three years, amazingthings have been happening in hard disks technology and the priceshave been dropping while the quality has improved.

How much data the drive will store is something you can read in theadvertisement. Make sure the size of the drive is advertised informatted bytes. Sometimes raw drives are listed with theirunformatted sizes. The formatting uses about 10% of the drive'scapacity.

Future graphics environments will use lots of disk space to storethe larger programs that will be written to use the larger memorieswe now have. Windows and its files and fonts takes more than 10megabytes on my machine. I think a minimum of 40 megabytes will beneeded on a 386 machine and you may find you need to add still morestorage space once you start using more and bigger programs.

PRINCIPLE # 1: You can never have too big a drive.

How quickly the drive's heads can be moved is something you canread in the advertisement. Average access time or average seektime is what is usually quoted. A slow drive is 85 milliseconds,an average drive is 28 milliseconds and a fast drive is in the midto low teens. Buy the quickest drive you can afford. You willneed a fast drive to move all the data that future environmentswill require.

PRINCIPLE # 2: You can never have too quick a drive.

MFM, RLL, ARRL and ERRL are ways of storing data onto a hard disk'ssurface. MFM usually results in a drive that has 17 512-bytesectors per track. Since most drives spin at 3600 revolutions perminute, the maximum theoretical transfer rate that you could everhope to get is 17 sectors times 512 bytes per sector divided by thetime it takes to spin one revolution (1/60th of a second). The RLLvariants allow more sectors per track to be stored and thusincrease theoretical throughput (typically 27 but up to 83 sectorsper track).

Maybe I should note here that ESDI and IDE drives often use RLL asa way of encoding data. So you can buy a ESDI RLL drive with"ESDI" defining how the drive and controller talk to each other and"RLL" defining how the data pulses on the disk will be interpreted.But, all things being equal, the more sectors per track, the fasterthe data can be transferred.

PRINCIPLE # 3: More Sectors Per Track Good

Likewise, the more heads that there are, the more data that can betransferred before you have to mechanically move those heads to thenext track.

PRINCIPLE # 4: More Heads Good

But what if the controller isn't fast enough to keep up with datamoving under its heads at that rate? Then interleaves areinserted. Interleaving at 2 to 1 will reduce your throughput by50%. 3 to 1 by 66%. Since 1 to 1 interleave controllers areavailable for very little more than slower controllers, there is noreason today to buy anything but a controller capable of a 1 to 1interleave.

Even if you are going to use your old ST506 MFM or RLL drive in thenew machine, consider getting a 1 to 1 controller. Without the 1 to1, you will have a machine that seems slow just because it isalways waiting for data.

GUIDELINE # 1: Interleave at 1 to 1 only

Once the data is read, the controller must transfer data to thecomputer's bus before it can get the data into memory. The widerdata path it can use (the more bits it can send at one time) thefaster it will be. There are some 8 bit SCSI interface cards andmany 8 bit MFM controller cards but they all are slow. Get nothingbut a 16 bit controller.

GUIDELINE # 2: 16 bit interface at least

Most of the disk reading we do is sequential. Once the readingbegins, it would be nice to be able to store all of the sectorsfrom that track into the controller's memory so that the next timeyour program asks for a sector, the controller will not have to doa physical read but can just transfer the data in a singlemillisecond. A ST506 MFM or RLL controller does this by havingenough memory to allow full track buffering.

Almost all drives of the ESDI, SCSI and IDE variety have full trackbuffering within the drive electronics. Makes no difference whereit is, controller or drive, just so long as it is there somewhere.

PRINCIPLE # 5: Full Track Buffering is Good

Some controllers and or drive electronics are able to low levelformat the drive so that not all the tracks start in the sameplace. They offset the position of the first sector of each track. It takes time for the drive electronics to terminate the read ofthe last sector on the previous track, switch heads and beginreading data from the next track. Head skewing allows enough offsetin the positioning of the sectors so that the sectors will be in aposition to be read just as soon as the drive electronics can getready to read the data. There is a similar skewing possible toaccount for the amount of time it will take the heads to be movedfrom cylinder to cylinder. This ability is called cylinderskewing. Together, these skewing techniques can increasethroughput by 5 to 10%.

PRINCIPLE # 6: Skew is Nice

Some controllers and drives read more of the data we are liable toneed than just a track worth. They have buffers bigger than justa full track buffer. And they do the reading while we areprocessing the data we originally asked for. The seeking andreading that goes on is totally transparent to the using program. This is a technique used only in high end controllers and drives.

PRINCIPLE # 7: Read Ahead Logic is Good

How fast you can read and write is limited to how fast the sectorsof the drive can rotate the area to be accessed under theread/write head. While 95% of all hard disks rotate at 3600rpm,some very high performance ones rotate faster (3707 and 4000 arespeeds I have seen).

PRINCIPLE # 8: Fastest Spin is Better

Manufacturers have so many different designs that you must becareful not to judge the reputation of a drive on the basis of areport on the reliability of a different product line the samemanufacturer builds. Reliability is a hard thing to determine. Just because your friend has one and his works, should you buy one? Is that a valid statistical sample? I guess the best place to seethe results from a large sample is the article on page 329 of the9/25/90 issue of PC Magazine on component reliability and thesurvey they had done of their readers. It isn't perfect but it isliable to be better than a sample of one. Make sure you read theparagraph on the appropriate series of drives.

GUIDELINE # 3: Know the Reputation

Caching is a technique for storing recently accessed data in memorysomewhere so that if you need it again soon, you can get it frommemory at memory speeds rather than having to read it from disk. Caching can be done on the controller, in motherboard memory (DOS's640k, extended or expanded) or both places. Should you buy acaching controller? Yes and no.

There are two types of caching controllers, one type with a smallamount (usually 32 or 64 k) of cache onboard the controller and onetype that allows several megs of memory on the controller.

With the first type, you can also use a secondary cache located inmotherboard memory. With the second type, you generally can't.

The first type of caching controller costs little more than a"normal" controller and the caching provides little more than fulltrack buffering. So buy that kind. With motherboard memory socheap, it makes little sense to me to buy an expensive ($500 toover $2000) caching controller when you can buy 4 more megs ofmotherboard memory for $200 and a program for $40 that does thesame thing. There is an excellent review of caching controllers inthe January 1991 issue of Byte which comes to this same conclusionand should be read if you are looking for maximum file transferspeeds.

If you must buy a true caching controller with large amounts ofmemory on the controller, consider if you want "write- through" or"write-later" caching. Write-through systems write the data tocache and also immediately to the physical drive. So, in case ofa power failure, your data has been written. But "write-through"caches are slower than "write- later" caches in write operations. "Write-later" caches write the data to cache and then later whenthe cache is full or when the heads are passing over where the datashould be written, the data is written out. Some "write-later"caches have battery backup so even if your machine goes down, thedata will be written out to disk the next time you boot yoursystem. Only you can determine the amount of speed you are willingto give up in return for increased safety. You know how reliablethe power is in your home or office. I have used a battery-backup"write-later" controller for several years with never a problem.

GUIDELINE # 4: Cache in motherboard memory, it's cheaper!

When you buy your subsystem, make sure that you receive a drive andits manual, a controller or interface card and its manual, cablesfor all the floppies and hard drives and tape drives the controlleris ever going to handle, mounting rails for the drive if your caseneeds them and if you don't already have them, 5 and 1/4 adaptorcages for any 3 and 1/2 inch drives you will be mounting in 5 and1/4 inch mounting holes, colored faceplates of the appropriatecolor, mounting screws and any software (diagnostic, partitioning,device drivers etc.) you will need to install the subsystem.

PRINCIPLE # 9: Get all the pieces

If you buy from one source, you can demand that the controller anddrive will work together and, if there are problems, you will haveonly one place to go back to.

GUIDELINE # 5: Get it all at one place

Most controllers will support two hard drives and two floppies. Some only the two hard drives. There is no real advantage toseparate controllers and two of them will cost more, take up morespace and create more heat. So buy the 2 floppy/2 hard version.

GUIDELINE # 6: Buy an Integrated Controller

Think about doing backups when you configure your system. Using afloppy backup program, you will want to use two alternating drives(one writing the data while you change the disk in the other one). Alternating drives must be of the same drive type (360kb, 720kb,1.2meg or 1.44meg) which means that you can't have a system haveboth 5 and a quarter and 3 and a half inch drive types so you canread all types of floppies without paying a big penalty in backupspeed.

Get a tape backup if you are getting a bigger drive. Who wants toload 100 1.44 floppies to do a backup? With a tape drive, I starta backup and go to lunch. Under these conditions, I do backups. If I had to fumble with hundreds of floppies, I would seldom (oh,be honest Mike, the word is "NEVER" !) back-up. There are tapedrives that connect to a three connector floppy cable, that fit ina 3 and a half inch mounting and that will back up about120megs/tape. I Use the CMS Jumbo. And they only cost about $300for the tape drive and $25 for each tape. If you get one of these,you will need a different type of floppy cable so ask about it.

GUIDELINE #7: How ya gonna backup

Manufacturer's guarantees sometimes begin when the manufacturerships the drive to the distributor who then ships it to the dealerwho then ships it to you. Make sure you know who is guaranteeingthe subsystem and for how long. Get it in writing.

GUIDELINE # 8: Know the Guarantee

The WD1003 controller IBM used in the early ATs set the standardfor what commands would be accepted by the controller and what thecontroller would do when it received a command. Your best bet isto buy only a subsystem that is compatible with the WD1003standard. That way, you'll have no chance for softwareincompatibilities. This is particularly important in the OS/2 andUNIX world but matters with some drive utilities in the DOS worldtoo. But don't buy the WD1003 itself. It would work but it isreally too slow for a 386 (It fails rules 5 and 7).

PRINCIPLE # 10: WD1003 compatibility is a must

You need your computer's memory for things other than devicedrivers. Some disk subsystems need device drivers to providesupport for large partitions (> 32 meg) or drives with greater than1024 cylinders (DOS has limits of 1024 cylinders, 16 heads and 63sectors. This can be "overridden several ways. "Drive Splitting" isa hardware feature of some top-end drives that allows a singlephysical drive to appear to DOS as two physical drives. "Translation" is a hardware feature of a controller or drive thattransforms the physical geometry of the drive to somethingacceptable to DOS. Say you have a 2048 cylinder drive with 8heads. Translation could make DOS think the drive had 1024cylinders by 16 heads. The third way to overcome limits is throughdevice drivers. Will they be available for OS/2 or UNIX for yourhardware if you want to go that route someday? See if you canavoid device drivers or at least buy drives and controllers thatalready have device drivers available for the software you might beinterested in someday.

GUIDELINE # 9: Device Drivers are Bad (but sometimes necessary)

Drives use as much as four times the power they take when runningnormally when they are powered up. Some controllers have a featurecalled "power-sequencing" that allows them to power up multipledrives with a bit of time in between each power up. This allowsthe power supply to be smaller and to not receive as large a surgefrom multiple drives powering up at the same time. If you expectto have more than two hard drives (via SCSI or secondarycontrollers), this is a nice feature to have. But you don't need"power-sequencing" in a normal machine and we are urging adequatepower supplies of the 200 watt or bigger variety for our buildmachines.

GUIDELINE # 10: Power Sequencing Nice at the top end.

ST506, SCSI, ESDI and IDE are standards for passing signalsbetween the drive and the controller/interface-card. SCSI, ESDIand IDE put more of the intelligence on the drive itself and lesson the controller than did the ST506 interface originally used onthe XT. (There is a good article on page 427 of the 10/30/90 issueof PC Magazine describing the techniques these three use to achievethroughput beyond that of the original XT's ST506 interface.)

SCSI is a standard that allows multiple (7 or more) drives andother peripherals to all connect through a single adaptor. SCSIhard drives, CD ROMs, and tape drives can all share the sameadaptor. SCSI is not a new standard but it is one that seems to besubject to a lot of configuration difficulties. Not that there isanything wrong with SCSI, it is just that the standard is ambiguousand different manufacturers have interpreted the standard indifferent ways. So you need to be particularly careful when usingSCSI components that they will plug and play together. I see lotsof BBS users reporting compatibility problems. Despite thiswarning, there are millions of happy SCSI users. If I were doingSCSI, I would buy all the parts from one dealer and I would get asubsystem guarantee that all the components were compatible inwriting. And I would not use any of the cheap 8-bit SCSI adaptors. Get a good 16-bit adaptor.

The choice of which of these other two (ESDI and SCSI) to use isless important than in insuring that you follow the other rules wehave mentioned above. I have used ESDI and IDE and can't tell thedifference. Both are available in versions that will COREtest atbetter than 900 k/second. Both are available in versions that willdo half of that. If you follow the rules we have talked about, youwill find yourself getting one of the faster versions.

The newer drives almost all use IDE, ESDI or SCSI interfaces so youwill get higher performance and higher value with one of thesethree. ST506/MFM and ST506/RLL work fine too, they are just slowerthan the newer ones.

You know better than I what you can afford and what you need. Ihope we have helped you understand a bit about hard drives and whatis available and what questions to ask. If you need additionalhelp, there will be experts at the pre-build session to help answeryour questions and the MIX is always available for questions (usethe HW conference). Good Shopping!

The WD1006/ST4096 combination COREtests at 450k/sec. But MFM isslower and more expensive per megabyte than any of the otherconnection types so buy MFM components only if you are adding anadditional drive to an existing subsystem (And even then considerselling your old disk subsystem and getting a new fast one. Iwould hate to see your brand new 386 slowed to a crawl by a slowdrive interface!). The RLL version of the WD1006 1 to 1 controllerand the MR535 drive cost the same as the MFM versions and give you26/17ths more storage and 26/17ths more speed. Stay away fromSeagate ST2xx series drives. While any manufacturer can have a runof bad drives, this series (especially the ST-238/251/277 drives)seems to suffer from a pair of design defects that cause motorlockup and striction (heads sticking to the platter).

Though not based on personal experience, people I trust havereported excellent results with this controller. It seems to befaster than the competition (above 1 meg with a normal machine andup to 1.4meg reported with a bus speed of 14mhz) and creates noproblems with installation and compatibility.

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All prices are current mail order and may or may not includecables, mounting brackets, adaptor kits, rails, s/w, taxes,shipping, installation documentation, a help line, etc. They arelisted to give you an estimate of what various levels ofperformance will cost. COREtest is a widely used drive performancemeasurement program. COREtest figures will always be lower thantheoretical figures for a drive due to DOS overhead.

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About the Author: Mike Focke works for HFSI, a systemsintegration company that sells exclusively to the US Government. His published articles include reviews of hard disk controllers forthe XT and AT class machines, an article on disk optimizationtechniques, reviews of Disk Optimization S/W, etc. He is afrequent participant on several nationally echoed Hard DiskConferences. He is currently designing and testing a hard diskdiagnostic program (DTST300.ZIP when it appears on the bbs).

I agree with virtually everything that Mike Focke suggests inDRIVEBUY.ZIP, but I have a few more things to add and a few furtherexplanations. These comments reflect my experience buying harddisks with severe budget constraints over the last 5 years. Exceptfor the disk speed tables, the comments contained here are somewhatless hard fact and more interpretation of the facts than in MikeFocke's "Principles" and Guidelines. My opinions are not alwayscorrect. Questions or corrections can be directed to me in the HWconference of the PC User's Group Mix. (301-738-9060)

One important principle to follow to avoid getting burned isto avoid the leading edge of hot hardware. I tend to wait untilsomething has been out for a while, in the hopes that the earlybugs will have been cured, and the gadget has establishedsufficient market volume so that it will be a long term success andnot a dead end. Owning orphaned hard disks is like driving withouta seat belt in the Dodgem cars. It may not matter, but bumps arecoming. For example, I was unwilling to buy IDE drives 6 monthsago, but now will buy nothing else in the 80 to 200 megabyte range. The IDE interface is now clearly going to be the way of the future. If I were richer, or working in a for-profit environment, I wouldtake more chances with new stuff from the large vendors, becausePCs are so fundamentally productive.

Another example of an important technology which is not yetready for prime time is the new hardware compression cards. EXPANZ! has been out for 4 months and the STACKER is advertised butnot quite yet shipping. Both of these manufacturers provide a cardwith a CPU dedicated to executing a disk compression routine forabout $200. The adds claim "double your disk space". Thistechnology promises to provide a cheap way to increase the capacityof your current disks by 50% to 75%, probably, within a year or so. But there are clearly compatibility problems with EXPANZ!, andStacker is not shipping in bulk yet, if shipping at all. Thefeasibility of using these things reliably without spending undueamounts of time becoming an expert in new levels of complexity isnot yet established.

Also, remember that computer hardware and software are gettingmore powerful at about 20% to 30% per year. Which means what makessense this year, might require some changing around next yearbecause of the coming technological improvements in PC's.

DISK SIZE AND UPGRADE STRATEGIES

My rule of thumb is to try to get at least TWICE the diskcapacity that will be immediately occupied. For serious dataprocessing machines that rule of thumb is too low. Human time istoo valuable to have someone spending 1/3 of their day trying toshrink their files to make space to do something else. Also, itseems to take 2 hours of time to put in a new hard drive and get itformatted and usable, no matter how many times I do it. On myfirst time with a different disk type it takes 3 or 4 hours. So Idon't want to do this very often.

Application software packages are growing in the disk spacethey occupy. This also means your software is more dependent ondisk reading, swapping in different parts of the program as you dothings as well as reading your own data files. For example, WordPerfect 4.2 occupies about 1.1 megabytes of disk space, but WP 5.1occupies about 3.8 megabytes. Likewise, Quattro consumes 1megabyte but Quattro Pro consumes about 4 megabytes. In order tohave pop-down menus and more options, fonts, etc, commercialsoftware is consuming more and more disk space. Therefore, disksize and speed are becoming even more important.

I WOULD RECOMMEND THAT AT THE MINIMUM YOU GET A 66 MEG RLL ORAN 80 MEG IDE DRIVE IN ANY 386. I favor 1/2 height drives, andonly one initially, so that I have the option of getting a seconddrive of equal or bigger capacity next year. Remember, thestandard AT CMOS Setup allows only 2 hard disks in a singlecomputer. The only standard way around this is with SCSI, whichallows up to 6 or 7 SCSI drives hanging on the single SCSIcontroller (if they are all compatible?) but only 1 non-SCSI harddisk on a second hard disk controller. There may be some otherspecial ways around the 2 drive limit, but they are not widelyused, and therefore I would be scared to use them because ofcompatibility problems.

I suggest the 66 megabytes as a minimum, because you can geta 66 meg RLL disk with an RLL controller for only about 60$ morethan the standard MFM 42 megabyte disk drive. The RLL 66 meg driveis 20 to 50% faster than the MFM and has 50% more capacity. However, it is not yet clear whether or how one can install an IDEdrive in a computer that already has an existing MFM, RLL, or ESDIdrive. So, the choice to get an RLL drive, may imply getting ridof that disk drive and controller if you subsequently need morethan about 130 megabytes of disk space. SCSI drives can usually beinstalled to co-reside in a computer along with any single harddisk of a different type. So, if you anticipate going to more than130 megabytes eventually, I would start off with an IDE 80 or 125,and plan on adding a second IDE drive later. If you anticipategetting to more than 400 megabytes of disk space, you might bebetter off starting with a 330 or 660 megabyte ESDI drive. But ifyou were buying 660 megabytes now, a single full height 660 wouldcost about 1900$, while two 330 megabyte ESDI drives with the sametotal space would cost 2800$ or so.

IDE versus ESDI versus SCSI. First, each protocol requires its own type of disk controller,and most disks will work with only one of the protocols. Second,there are two bottlenecks in disk speed--the controller protocoland the hardware speed in the drive. The MFM protocol is slowestbecause its controller speed limit is about 500K characters persecond, and its disks contain only 17 sectors per track, limitingthe amount of data that can be read in a single revolution of thedisk to 8.5K. RLL raises the controller speed limit to about 800Kper second, and has 26 sectors per track, meaning that in the samerevolution of the hard disk it can read 13.5K characters. TheSCSI, ESDI and IDE protocols all allow many more sectors per track(34 to 78), and have a current controller speed limit in thevicinity of 1 Meg/sec. Some of the 330 and 660 megabyte ESDIdrives can go to 1.5 megabytes per minute or 2 megabytes per minutewith the correct controller.

For drives between 80 and 200 I will be buying exclusively IDEprotocol drives. The IDE drives have really pushed the price/performance barrier in the past 6 months to new levels. The newSeagate 71 meg IDE drive is about 350$, and it is 2.5 times asfast as the 330$ Seagate 296N SCSI 84 meg when the latter is run onthe cheap Seagate 8 bit SCSI controller. I have seen the 200 megMaxtor and Seagate IDE drives advertised for 775$, and they onlyneed a 20$ to 50$ card to interface to your computer--whilecomparable performance SCSI or ESDI controller cards cost from 160to 300$, and the ESDI/SCSI hard disks tend to be somewhat higherpriced also. The IDE drives are usually 1/2 height, 3 1/2 inch,low power (1 to 10 watt) drives and they are recent engineeringdesigns. In contrast, many of the 150 meg ESDI drives are fullheight, 3 or 4 year old designs that consume 25 watts of power,generating more heat and more vibration. There are also 1/2 heightESDI's in the 150-300 meg range, which tend to be newer designsthan full height drives.

As of December, 1990, you must use either SCSI or ESDI above200 megs with the exception of 2 drives from Seagate. Above 200megs, most drives are either of size 330 or 660, and most areavailable as either ESDI or SCSI. There appear to be fewercompatibility problems with ESDI, but the SCSI protocol is going tobe pushed in lots of larger mini-computer and mainframeapplications (Business Week, Dec 25, 1990), and performancebreakthroughs in big machines in the SCSI format may move down toPC's. Vendors are now selling the same 5 1/4 inch SCSI hard disksfor use in mainframes, minicomputers, or PC's. A new SCSI protocol(SCSI-2) is coming that increases the disk I/O speed to 4 megabytesper second, but it is not quite here yet, and may still be a yearor two off.

A second new SCSI protocol that is much more expensive, butpromises vast performance improvements is the Drive ARRAY/DiskStriping approach. This is what will soon be eating into themainframe and minicomputer disk market and is available for CompaqSystemPros now. It runs 2 to 10 different physical disks as onelarge disk from the operating system's point of view, by strippingeach character into the 8 bits and writing part of each characterto a different disk. This has radical speed advantages, and with10 different disks, you can achieve new levels of redundancychecking. A 10 disk array of 900$ 200 meg Conners 16 ms driveswould appear to the computer as an 8x200 = 1.6 gigabyte disk withan average access time of 2.0 milliseconds. 2 of the disks would bedevoted to error checking and data redundancy. Unfortunately, thecontrollers for these gadgets now cost 5,000 to 20,000$.

ESDI has a slight compatibility edge in PC's now because1)there is greater incompatibility between SCSI controllers andspecific SCSI devices, and 2)the SCSI drives usually require use ofa separate address space in High RAM between 640K and 1024K. Thiscan lead to conflicts with other device drivers, network cards, ROMShadowing, etc. This should be solvable, but it is one moreconstraint on the use of addresses in the upper range and one moreconflict to dodge in setting up 386 memory managers and multi-tasking.

The one advantage that SCSI has over ESDI and IDE drives incompatibility is in non-standard ROM BIOS machines or in older ROMBIOS machines. That is because the SCSI drives are not evenentered into the setup but ESDI and IDE drives are entered in thesetup. Therefore, you have to have a drive type in the list of 14to 48 physical drive types that matches the drive you buy, or youhave to have a "customizable" drive type in the ROM BIOS, as thenewer AMI, Phoenix, and AWARD ROM BIOS's do have. If you don'thave a customizable drive type (where you can type in the number oftracks per head, number of heads, and number of sectors per track),then an IDE drive can still be used without a device driver as longas the disk is not larger than the largest total amount of diskspace available in the largest fixed drive type available. The IDEdrives have sector remapping built into the drive to translate fromthe CMOS table to the drive's actual physical configureation. Youmight loose some disk space here. Another alternative is to use ahard disk device driver such as On-track's generic DiskManagerversion 4.2, which handles translation from the CMOS drive typesinto a layout that the drive needs. With an older ROM BIOS thetranslation from a device driver such as DiskManager is more likelyto be necessary with an ESDI drive than with an IDE drive.

DISK SPEED

The faster the better. But how fast and how do you judge? First, using an 8 bit MFM or RLL controller in any AT or 386 isdisastrously slow. They require an even larger interleave than the3 or 4 to 1 they usually require in an XT. So a standard AT 2/1hard-disk controller will provide a radical improvement in speed,and can probably be bought used for 30 or 40$.

However, for 100$ you can buy a new 1/1 interleave controllerwhich does provide substantially faster disk I/O than the 2/1 for2 reasons: first is the 32K cache on the controller and second is1/1 versus 2/1 interleaving, which means that the computer can reada whole track in one revolution of the disk instead of 2. One wayto read interleave numbers is the top number is the number of diskrevolutions required to read all of the data on a single track.

If you are doing data base processing on files large enough tooccupy more than one track on your hard disk, then skewing, trackto track access time, and average access time become moresignificant. The average access time doesn't matter as much as thetrack to track--IF YOU KEEP YOUR DISK DEFRAGMENTED--and theopposite is true if your disk is fragmented. There can bediscrepancies between track-to-track and the average seek time tofind a random other track. Unfortunately, track to track times arenot advertised as much as the average access time, and it isdifficult to find out which controllers support skewing. Thus, nomatter what kind of disk you have, if you are doing substantialdata processing on large files you will need to use the disk de-fragmenting program available in such widely sold commmercialsoftware packages as Norton Utilities, PcTools, PCKwik Power Pack,etc.

There are a number of speed testing programs, but none areperfect. There are "rifle shot" testing programs that test thehardware's ability to read a specific sector and develop hardwarespeed measures, and there are other testing programs that read andwrite a 500K to 1meg test file and measure the elapsed time. Bothare useful pieces of information though neither is perfect measuresof how well your own application software will work with differentdisks.

Performance measurement is like looking at the world throughmultiple layers of a veil. You are rarely sure whether what youobserve is an object on the distant horizon or a fly that landed onthe veil. The best speed tester available seems to be CORETEST,which is available on bulletin boards. PC MAGAZINE also hasproduced a standard performance testing program that is on bulletinboards. NORTON, SPINRITE, and CHECKIT are commercial softwarepackages that contain disk speed tests, as part of their package. Beware, however, that SPINRITE and CHECKIT's disk speed measuresare pure measures of how dense the data is recorded on a singletrack, and not composite measures of how fast the disk will be ableto access data files under DOS. In other words the speed measuresin CHECKIT and SPINRITE ignore track-to-track and average seektimes. NORTON and CORETEST seem to reflect the head movementspeed as well as data density.

The following tables summarize some disk speed comparisontests using the Coretest v 2.92 speed testing software. Readersshould be cautioned that no speed testing software is perfect. Coretest is probably the best available but it may be biasedagainst or for particular drive/controller combinations. Coretestresults are not sensitive to the difference between AT's and 386's,or to clock speeds of the CPU. There is some variability, however,in individual drive performance within model types. In fact thereis likely to be much more variation in the performance of 2different disk drives of the same model number than in most otherparts of the computer. Finally, how a disk is formatted mayinfluence disk speed, and 386 memory managers such as Windows orQEMM do slow down disk I/O. Many of the disks listed below belongto other people, and I do not know for sure how their machines wereset up. I do know that they were not running disk cache's, otherthan the 16K to 32K RAM track buffer that is built into manydisk/controller systems.

The last column of numbers in Table 1 is recent mail orderprices. These are prices for naked drives, excluding controllers. Some vendors include cables, 3 1/2 to 5 1/4 adaptor kit and OntrackSoftware, and others don't.

For comparison purposes, I have also included some benchmarknumbers using coretest on some older Disk drives in AT machines andin XT machines in Table 2. Thus, if you are wondering how muchfaster some hot new drive will be than your existing drive, you canlook up in table 2 a drive similar to your existing drive, andcompare the speed numbers with those you are considering in Table1 above. Or if you know the Norton SI speed rating on your disk,you can find a drive in Table 2 with the same Norton SI, and readits Coretest Index number off the same line. Then Compare Coretestnumbers with those in Table 1.

I tend to examine both characters per second and the indexnumber. It is the index number that is probably the best summarycomparison number. If your current disk drive has a Coretest Indexof 3.0, and the disk you are thinking of buying gets a 6.0 on thesame measure in the table above, then that disk is approximatelytwice as fast as your current disk.

What do Tables 1 and 2 mean? FIRST, they illustrate the substantially higher disk speedsavailable with the IDE protocol and the RLL protocol than the bestavailable from 1/1 interleave on the MFM protocol. That is why Iwould now avoid the MFM protocol. I would avoid any MFM diskdrive. SECOND, most of the 80 to 120 meg IDE drives are not thatmuch faster than 66 meg RLL drives with the notable exception ofthe Western Digital 81 meg and the Seagate 1144A 125 meg drive. Soan RLL disk drive protocol is acceptable, if the total disk spaceyou need is not larger than 132 megabytes.

THIRD, The cheap Seagate 296N with the accompanying SeagateSCSI controller is the slowest disk drive tested in all of Table 1. It should be avoided, unless you need SCSI and cannot afford afaster controller.

FOURTH, all of the 200 meg drives are substantially fasterthan most of the rest of the disks, and they are cheaper permegabyte. So, if I knew that I needed 80 megabytes of storage now,I would be inclined to get the 200 rather than a 125 meg drive.

FIFTH, the ESDI results do reveal that there is somevariability in ESDI drive speed, but it is possible to get up to1400 characters per second with 330 meg ESDI drives and theappropriate $200 controller. So for maximum size and speed ESDI(and SCSI) do offer some advantages over IDE.

Finally, it appears that the WD drives are a bit faster thanmost of their competitors on this test. It is possible that thisapparent advantage would be reduced in real data analysis withlarger files, because these disks have larger (64K) caches builtinto the drive than the 32K cache built into many of the otherdrives. On the other hand the advantages may be real. A muchlarger effort would be required to test that hypothesis.

For new machines, the IDE protocol is probably the bestchoice, unless you 1) have an existing disk controller and diskthat you want to use, perhaps with a second new disk, 2) youanticipate growing to more than 300 to 400 megabytes of data within12 to 18 months, or 3) this is a network server. In latter 2cases, you are probably better off with a full height 660 megabytedisk drive from the beginning. But barring those three exceptions,the IDE protocol now delivers more disk capacity and speed perdollar, and it has a lot of growth potential in half height diskdrives, up to 330 megabytes on a single disk. (Conner Peripheralshas just announced a 510 megabyte IDE drive).

The choice of disk drive protocol, RLL versus IDE, versus ESDIor SCSI is important because it constrains your future upgradechoices and costs. It is difficult if not impossible to have 2hard disks of different protocols in the same computer among theMFM, RLL, IDE, and ESDI protocols. A computer with one of thosedisks can usually also have a SCSI disk, but a computer with one ofthe MFM, RLL, IDE, or ESDI usually cannot also have a different oneof those 4 types also. Thus, if you get an RLL now, and need morespace next year, you will have to get either an RLL or a SCSI. One caution about IDE disks is that most IDE disks above 70megabytes will probably require that your CMOS have a "customizabledrive type" in the setup. Older ROM BIOS chips may be missingthat, or may not have a customizable drive type that works. Thusif your BIOS is older than early 1990, you may need a new ROM BIOSchip. One specific brand, Conner Peripherals, was reported to beincompatible with the AMI BIOS dated before April 1990. In theabsence of a customizable drive type, On-Track's generic DiskManager version 4.2 might solve the problem, but that might alsocause compatibility problems with Windows 3.0.

BRANDS-DISKS

I tend to stick with the market leaders in volume for severalreasons. First, there are enough of them around that I can getgood prices and will be able to get them repaired cheaply, ifnecessary. Second, why are they the market leaders? Theirhardware must be relatively good. Third, everything works with thehigh volume disks. New software and hardware developers design forcompatibility with the largest number of machines they can achieve. They design for compatibility with the highest volume producersfirst.

For an RLL 66 meg half height drive I also have heard verygood things about the Mitsubishi MR 535. The Toshiba is $20 to $40more expensive but it has more heads and is a 3 1/2 inch drive. The Seagate 277r is of the group of Seagate drives which have hadmany reports of "stiction", where the drive will no longer startup, but once started will continue to spin. One disadvantage ofthe Mitsubishi MR535 is that you often have to use a "customizableDrive type" for it. Even though there are 3 different drive typesin my AMI BIOS with 977 tracks per head and 5 heads, none of themwould work.

Right now the market leaders in the IDE protocol are ConnerPeripherals, Seagate, and Maxtor, with Western Digital probably4th. Each makes a 40, 80, 100, and 200, and Seagate also makes a125, 160, 240, and a 330. The 200's have faster seek times thanthe 80's and 100's, and usually have denser recording of data, withmore 512 character sectors per track, so they should produce fasterthroughput.

Seagate also has 2 overlapping lines in the 90 to 200 megabyterange, with the more expensive ones having faster 15 Ms seek timesand the slower ones 20 millisecond seek times. This has moreserious implications for the do-it-yourselfer than might bethought. The 15ms access time disk drives are the line of diskdrives developed by Control Data Corporation, whose disk drivesubsidiary Seagate bought 15 months ago. The drives were designedto be installed in many mini-computers and are generally accordedhigh quality ratings. However, they were not designed to beinstalled by a do-it-yourselfer in PC and they are somewhat moredifficult to install physically.

At least the 200 meg Seagate 1239A that I bought last week wasmore difficult. It is the first disk drive I have ever seenwithout a clear mark on the data connection end of the disk driveas to which end was line 1 and which end was line 40. Also, it wasthe first disk I have seen in 50 whose screw holes did not line upwith the standard locations for physical attachment. To compoundthe insult, Seagate apparently does not routinely include a manualwith the disk drive. In contrast, a 20 millisecond Seagate 1144a,one of the Seagate line as opposed to the Seagate ownedCDC/Imprimis/Swift line, was the easiest disk I have everinstalled. The absence of a manual can be solved by downloadingSeagate manuals from their bulletin board. But the differentheight for the screw holes can cause this half height drive tooccupy 2 half height bays, or might make it difficult to firmlyattach in a 3 1/2 inch bay.

For disks over 300 megabytes there seem to be 5 or 6 largemanufacturers, with the three leaders being Maxtor,Seagate/CDC/Imprimis, and Micropolis. Hewlett-Packard, Fujitsu,and Microscience also compete here, as do a few others, includingCore. The first 6 manufacturers produce 660 meg drives withessentially identical advertised performance and similar prices. Core drives are more expensive, but I think they provide a 5 yearreplacement warrantee, while the others come with a 1 year or 2year (Maxtor) warrantee.

BRANDS -CONTROLLERS

First, for IDE drives there is not much to the controllers. Some new computers have the IDE interface built into themotherboard and others require a separate card, but in the lattercase there really is not much intelligence on the card. The worksare on the disk drive. So controller performance probably onlymatters for SCSI, ESDI, RLL, and MFM.

If you are concerned about Novell, Os/2, or Unix, Mike Focke'sdiscussion about WD1003 compatibility is extraordinarily important. It may be important for other things too, but it may not. WD isWestern Digital, and most of the other controller manufacturersmentioned here make controllers which achieve WD1003 compatibility. Often the other brand controllers also have a non-compatible modethat is useful in some circumstances, or might provide higherthroughput.

WD makes both SCSI and ESDI controllers as well as MFM and RLLcontrollers. However, in among SCSI vendors Always, Future Domain,and Adaptec seem to be the market leaders. But it is crucial toobtain a controller that works with your disk. For SCSI it isimportant to buy controller and disk from the same vendor. In theESDI arena the most widely sold brands appear to be the Ultrastor12F, WD 1007v.SE2, and Adaptec. WD has just come out with a WD1009ESDI controller which is rumored to be quicker. Anothermanufacturer that has long produced a good line of MFM and RLLcontrollers is DTC, which is a subsidiary of Qume. Note, many ofthese manufacturers have a lower priced model which does notsupport 1/1 interleave or does not have a full track buffer on thecontroller. It is penny wise and pound foolish not to get thecontroller with the 32K or 64K buffer/cache built into thecontroller.

I stay away if at all possible from no-name controllers oroff-brand controllers. A few dollars here can make a world ofdifference to performance and compatibility in the future. Thereare often new brand names with some "hot" new board, but in diskcontrollers I tend to be most conservative.

VENDORS.

There are a wide variety of vendors of these disks. Many mailorder places advertise in PC MAGAZINE, the PC SHOPPER, PC WEEK orINFO WORLD, with PC SHOPPER probably having the best disk prices. If you are buying a big disk, you ought to check out the prices inthe adds, and in Softwarehouse, the local superstore in TysonsCorner, before negotiating with other retailers. Many otherretailers will probably come close to or beat the mail order price,and they may help you format the disk and provide valuableadvice/handholding. If they don't, or you don't need thatassistance, why pay more?

PC SHOPPER has a vendor index, so you can look up what page theadds are on. I have bought from 4 or 5 of these vendors, as wellas local suppliers, but have not bought from all of them, and Ihave bought from other vendors not listed here as well as localretailers.

I suggest buying on Mastercard, and stopping payment if theydon't include the manual for the disk and the controller. Twoyears from now, when you want to reformat to use this hard disk onyour new 586 and quad speed disk protocol, you will probably needto change a jumper setting on the controller or the disk. Somevendors include On-track formatting software with all big disks,and others don't. So ask what is included. Also, some includecables, rails, and some include 5 1/4-3 1/2 mounting kits, whileothers charge extra. Also one needs to ask about warrantee, andtime allowed for returning hardware that is dead on arrival.

Compatibility with the WD 1003 standard can be a problem, becausethe limits in that standard were 1) 16 heads (or surfaces) perdisk, 63 sectors per cylinder, and 1024 cylinders per head. Whenthe standard was set unfortunately they did not anticipate thegreat increase in the material sciences and stepper motor controlof head location, allowing much denser data recording on magneticsurfaces. Most disks over 100 megabytes now use more than 1024cylinders per head, and some of the bigger disks are using up to 78sectors per cylinder. In order to achieve DOS compatibility, low-level formatting software has to do some sector-remapping, whichcan, depending on how it is done, cause problems with WD1003compatibility.

The IDE drives usually do not require a low level format--justpartitioning and high level formatting. Apparently they have thesector remapping built into the disk in ways that allow thecomputer to think it is a 1003 compatible device.

The ESDI's and SCSI's usually do require a low-level format,and how you do it can be difficult, primarily because of the issuesrelated to mapping sectors above 1024. Another issue regarding bigdisk drive partitions is how do you maintain a logical diskpartition over 132 megabytes. There have been a number of reportson the mix of difficulty doing defragmentation with either PCToolsv 6.0 or Norton v.5.0 on logical partitions greater than 132megabytes. With a 660 megabyte ESDI drive low-level formatted andpartitioned with Disk Manager Generic version 4.2, and high levelformatted with DOS 4.01, however, I have successfully defragmenteda 200 megabyte logical partition containing 190 megabytes of datawith PCTools version 6.0.

At this time, MS DOS 3.3, MS DOS 4.01 and DR-DOS 5.0 are theoptions. Microsoft's MS DOS 5.0 is due in April 1991, and islikely to dominate earlier versions of MS DOS and offer the sameadvantages currently in DR-DOS 5.0. DR-DOS is believed to be 100%compatible but it was done by a different company, and only a tinyminority of PC users have tried it yet. The manufacturer of DR-DOS5.0 is, however, a highly regarded operating system programmingfirm. They developed and distributed CP/M.

The principle advantages of DOS 4.01 are that it allows largerthan 32 Megabyte hard disk partitions-both in the boot drive and inupper logical drives. Secondly, it is compatible with Windows 3.0. The 3 disadvantages are 1)it is limited to 1024 cylinders and to512 Megs of storage per disk without using 3rd party software topartition, 2)DOS 4.01 uses more of the 640 K of DOS RAM for systemsprograms than does DOS 3.3 3)DOS 4.01 is a bit harder to installbecause of its 5 floppy disk shuffle installation.

DOS 3.3 forces the boot drive to be less than 33 megabytes,and all other partitions are also less than 33 megs, unless you useSpeedstor or OnTrack's Diskmanager to partition your disk. In thatcase, the boot drive is still restricted to 33 megs, but the upperpartitions can be larger. Disk Manager may achieve bettercompatibility with the WD1003 standard and may achieve worsecompatibility, depending on the particular disk drive andcontroller. There appear to be some potential conflicts betweenthe way some IDE drives handle their sector remapping and how On-track's DiskManager tries to set up the sector remapping. Bysector remapping here, I mean how the disk drive physical layoutwith greater than 1024 cylinders is mapped so that the computerthinks it has a 1024 cylinder disk with more heads or more sectorsper head than actually physically occur on the disk.

Recently, I have had some serious difficulties with an IDEdrive formatted with Disk Manager generic version 4.2 and DOS 3.3. Diskmanager handled the CMOS drive type differently than I wasexpecting, and I believe that the Diskmanager BIOS overlay thathandles sector translation may be incompatible with the sectortranslation built into this particular IDE drive, or at least theway that I have done it is not compatible. Therefore, I WOULDRECOMMEND THAT WITH THE LARGER IDE DRIVES, ONE USE DOS 4.01 AND USETHE DOS FDISK PROGRAM TO PARTITION THE DISK if you want disk drivepartitions greater than 33 megabytes. If you do not need partitionsgreater than 33 megabytes then either DOS 3.3 or Dos 4.01 will do,but one should probably use the DOS FDISK rather than Diskmanager. This may merely a problem with an old ROM BIOS, correctable by anupgrade, but the IDE drive is not functioning reliably using thegeneric DISK Manager.

Summary.

Microcomputer hardware is getting better at 20 to 30% peryear. Maybe what is available now will be good enough forever, butmaybe not. Current hardware selection that is cheapest and best todo the job now may imply higher future upgrade costs. Hard disksize and speed will be ever more important in running amicrocomputer to do new applications, because many new applicationsare now being designed to run with a hard disk, without muchconcern for usability with floppy disks alone. Think carefullyabout the trade-off between best performance per dollar now, andbest performance per dollar next year. Your hard disk needs willgrow.

For a 386SX or larger computer I would now buy, at a minimum,a 66 meg RLL drive or a 70/80 meg IDE. Because of the rapid paceof conversion to the IDE protocol, using an IDE protocol intially,even with only a 40 megabyte drive, will maintain greater optionsfor low-cost future upgrades, as long as your total disk spaceneeds do not exceed 400 megabytes. The IDE protocol seemsattractive now, up to 200/330 megs on a single disk. The 200's arenot only cheaper per megabyte of storage than smaller disks, butalso faster than smaller drives. If maximum disk size and speed isneeded ESDI or SCSI on 330 meg or 660 meg disk drives canapparently outperform the 200 meg IDE drives, though atconsiderably higher total cost but still lower cost per megabyte. Also, for networking, there may be substantial advantages toESDI/SCSI interfaces.

----------------------------------------------------------------About the author: Doug Wagner works at George Washington U. MedicalSchool as a statistician/health services researcher in a long termresearch project to develop a better measure of severity of illnessfor acutely ill, hospitalized patients. The research project hastransitioned from being the largest paying customer of Universitymainframe computing to using a VAX 780, to based entirely on PC'sfor extensive statistical computation and data base management. Doug Wagner can be reached in the Capital PC User's group MIXbulletin board (301) 738-9060 in the HW (hardware) or TRAINconferences.